This application relates to improvements in ovens of the type disclosed in copending application Ser. No. 06/177,174, filed Aug. 11, 1980, which is a continuation of application Ser. No. 038,129 filed May 11, 1979 both abandoned. The disclosure of each of these applications is incorporated herein by reference in its entirety for all purposes. These applications are assigned to the same assignee as the present application.
Although temperature control is critical to the successful cooking of food products, difficulty is encountered in heating the product uniformly to the optimum temperature because the heat required to obtain the desired heating varies with a number of factors including the volume and shape of the product, the initial temperature of the product, the porosity of the product, the moisture content of the product, and the composition of the product. For example, in a conventional oven, the cooking time and temperature for a five-pound roast which is shaped like a cube may be significantly different than the cooking time and temperature required to cook a two-pound roast cut to form a slice 1/2-inch thick. Similar difficulties are encountered in microwave ovens and forced convection ovens.
A variety of time and temperature control devices have been employed heretofore in ovens for cooking food products. Thermostatically controlled burners or electric heating elements are commonly used in combination with a timing device for cooking a product at a predetermined temperature for a predetermined time period. Probes have been used for measuring the internal temperature of a product so that heating of the product can be terminated either manually or automatically when the internal temperature of the food adjacent the probe reaches a predetermined temperature.
Microwave and surface heating devices have been employed in tunnel ovens such as those made by Cry-o Dry Corporation using a combination of circulating hot air and microwave to dry potato chips. Another tunnel application has been built by Litton Industries Inc. for Consolidated Foods Co. in Berryville, Ark., using a combination of steam and microwave to moisten and cook chicken. Tunnel ovens using microwave heating to heat rubber preforms are made by Gerling-Moore Co., San Francisco, Calif., and tunnel ovens to thaw meat are made by Ratheon Co., Waltham, Massachusetts. Microwave tunnel ovens with surface cooling and with surface steam heating are made by Les Micro Ordes, Paris, France.
Microwave tunnel type ovens have been employed for heating meals in hospitals. One such oven has been built by AFS Systems of Sweden, in which each portion of the food product is carried in a container and the container is in turn carried through the tunnel by an endless conveyor system. A plurality of microwave generators are disposed sequentially along the tunnel. A lever is used to turn each microwave generator on or off. As each container passes, it trips the lever to heat the food and, as the container moves past the lever, the lever swings back to its normal position to turn the generator off. The amount of radiation emitted depends on the time the lever is held in the "on" position. When transporting a series of containers through the tunnel the lever, held in the "on" position, will cause continuous emission of microwaves, which means the different food products along the conveyor are continuously heated by microwaves emitted from all of the generators. Accordingly, the heat supplied to each individual food product or portion is not controlled as much as desirable when different sized containers or different spacing of containers on the conveyor are encountered. This problem is especially acute when the successive increments of food products require different heating, such as when one meal has an entree and two vegetables and the next is salt free (which heats differently in microwave) and the next is a vegetarian meal.
Temperature control devices of the type heretofore employed are disclosed in a number of U.S. Patents including: U.S. Pat. No. 3,177,335; U.S. Pat. No. 3,431,399; U.S. Pat. No. 3,611,336; U.S. Pat. No. 3,884,213; U.S. Pat. No. 3,931,620; U.S. Pat. No. 3,974,696; U.S. Pat. No. 3,975,720; U.S. Pat. No. 4,086,813; U.S. Pat. No. 4,149,056; and U.S. Pat. No. 4,217,477.
The invention disclosed herein relates to improvements in apparatus for controlling ovens and particularly microwave ovens while eliminating the need to position probes and other devices in or on the food product.